1. Field of the Invention
The present invention relates to a differential amplifier and, more particularly, to a differential amplifier used in a balanced/unbalanced converter circuit.
2. Description of the Related Art
Generally speaking, balanced transmission is used in a circuit for communication. Balanced transmission is known to be highly resistant to noise and adapted to the circuit integration technology in that a differential amplifier not requiring capacitors can be used. In contrast, signal transmission in general, such as antenna transmission, uses unbalanced transmission and therefore requires a converter circuit for conversion between a balanced system and an unbalanced system. A merchant balun, using coupled line segments of a transmission line, and an active balun, using a differential amplifier, are generally used as a balanced/unbalanced converter circuit built in a communication IC. A merchant balun causes a chip area to be increased as a result of using an inductor corresponding to ½ of the wavelength of an operating frequency. In contrast, an active balun can be miniaturized since it usually uses a differential amplifier.
FIG. 1 illustrates the structure of a differential amplifier 10 for a balanced/unbalanced circuit according to the related art. The differential amplifier 10 includes FETs M1 through M3, a resistor R1, a resistor R2, resistors R5 through R8 and a capacitor C2. The sources of the FET M1 and the FET M2 are connected to each other and also to the drain of the FET M3. The FET M3 operates as a current source. The drains of the FET M1 and the FET M2 are connected to a power supply potential VDD via the resistor R1 and the resistor R2, respectively. An unbalanced signal is fed to the gate of the FET M1 via an input terminal 14. The gate of the FET M2 is connected to the ground or a VSS potential via the capacitor C2. The resistors R5 through R8 apply equal bias voltages to the gates of the FET M1 and the FET M2. With such a structure, the input unbalanced signal fed to the gate of the FET M1 is amplified by the FET Ml. Since a constant current flows in the current source constituted by the FET M3, a current 180° phase-shifted with respect to the drain-source current of the FET M1 flows between the drain and the source of the FET M2. Consequently, signals retrieved from a first output terminal 16 and a second output terminal 18 are of the same amplitude in potential and 180° phase-shifted with respect to each other, so as to form balanced signals (See reference (1) in the following Related Art List, for instance).